The use of global positioning systems (GPS) to determine the terrestrial position of a portable device is well-known in the art. For instance, U.S. Pat. No. 5,375,059 to Kyrtsos et al., U.S. Pat. No. 5,438,517 to Sermon et al., and U.S. Pat. No. 5,490,073 to Kyrtsos each describe a navigational system for vehicles utilizing the electromagnetic signals received from GPS satellites. The aforementioned patents (U.S. Pat. No. 5,375,059; U.S. Pat. No. 5,438,517; U.S. Pat. No. 5,490,073) are hereby incorporated by reference in their entireties.
A global positioning system works by utilizing a network of GPS satellites that continuously transmit signals to the Earth; the data transmitted by these signals includes the precise time at which the signal was transmitted by the satellite. By noting the time at which the signal is received at a GPS receiver, a propagation time delay can be calculated. By multiplying the propagation time delay by the signal's speed of propagation, the GPS receiver can calculate the distance between the satellite and the receiver. This calculated distance is called a “pseudorange,” due to error introduced by the lack of synchronization between the receiver clock and GPS time, as well as atmospheric effects. Using signals from at least three satellites, at least three pseudoranges are calculated, and the position of the GPS receiver is determined through a geometrical triangulation calculation.
When GPS signals are not available, the position of a portable device may also be calculated through other means, such as a dead-reckoning system incorporating an accelerometer. For instance, U.S. Pat. No. 5,606,506 to Kyrtsos and U.S. Pat. No. 6,308,134 to Croyle et al. each describe navigational systems integrating both GPS and dead-reckoning techniques. U.S. Patent Publication No. 2007/0260398 to Stelpstra further describes a device that calculates calibration parameters for its accelerometer while GPS data is available, enabling the device to determine its position exclusively using data derived from the accelerometer when GPS data is unavailable. The aforementioned patents and patent publications (U.S. Pat. No. 5,606,506; U.S. Pat. No. 6,308,134; U.S. Patent Publication No. 2007/0260398) are hereby incorporated by reference in their entireties.
Certain currently available GPS systems also utilize remote databases to store GPS related information, which is then communicated to a portable device. U.S. Pat. No. 6,222,483 to Twitchell et al., for example, discloses a GPS location system for mobile phones in which the GPS satellite information is stored in a database on a server accessed via an Internet interface. The aforementioned patent (U.S. Pat. No. 6,222,483) is hereby incorporated by reference in its entirety.
However, most commercial enterprises that rely on GPS based positioning systems use that information in conjunction with physical land tract information. For example, surveyors typically use GPS in conjunction with traditional geometrical calculations to mark land tract perimeter angles to produce a valid land survey. Further, construction crews hire surveyors to install physical markers on land tracts so that their equipment does not stray outside of authorized construction areas. And, logging operations typically use satellite photos in conjunction with GPS information to navigate harvesting activities in authorized land parcels.
Therefore, what is needed is an integration of current geo-positioning techniques with accurate land tract information so that land tract navigation would be facilitated and simplified.